1. Field of the Invention
The present invention relates to a novel high-strength austenitic steel and, more particularly, to a high-strength heat-resisting austenitic steel suitable for use as the material of a turbine casing and valves of a super critical pressure steam turbine which operates with steam of extremely high temperature and pressure, as well as the material of a reaction furnace of a chemical equipment which operates at high temperature such as, for example, styrene monomer synthesizing tower.
2. Description of the Prior Art
The current tendency of a shortage of petroleum resources and the rise in the price of the same have given an increase in the demand for improvement in the thermal efficiency of a steam power plant through the use of steam of higher temperature and pressure. The steam turbine of modern steam power plant operates at a steam temperature between 538.degree. C. and 566.degree. C. The turbine casing and valve bodies of steam turbines operable at such a high steam temperature are made from Cr-Mo-V cast steels which exhibit high resistance to heat. This type of heat-resisting cast steel, however, undesirably exhibits grain boundary slip at temperatures above 550.degree. C. and, hence, an extremely low creep strength. For this reason, this type of cast steel cannot be used at high steam temperatures above 600.degree. C.
Generally, heat-resisting austenitic steels such as SUS 304, SUS 316, SUS 321 and SUS 347 as specified in JIS (Japanese Industrial Standard) are used suitably at high steam temperatures exceeding 600.degree. C. More specifically, the steels SUS 304 and SUS 316 show a 10.sup.5 -hour creep rupture strength of 6 Kg/mm.sup.2 or less at 650.degree. C. Considering that a 10.sup.5 -hour creep rupture strength of 7.7 Kg/mm.sup.2 or higher is required under the steam condition of 600.degree. to 650.degree. C. and pressure of 316 to 352 atg., the steels SUS 304 and SUS 316 cannot be used under such a severe condition.
Japanese Patent Laid-Open Nos. 109421/77 and 158853/81 disclose addition of strong carbide formers such as Nb, Ti, Zr, V, etc. to heat-resisting austenitic steel to improve the high-temperature strength of such steel. These literatures, however, do not show or suggest any relationship between Al and N contents of the steel. The present inventors have found that these elements added to the steel show higher stability in the form of nitrides or carbonitrides than in the form of carbides, so that these elements tend to form nitrides or carbonitrides such as NbN, TiN, ZrN, Nb(C,N), Ti(C,N), and Zr(C,N). These nitrides and carbonitrides are substantially insoluble to the matrix. In addition, when a steel ingot of a diameter greater than 50 cm or of a weight greater than 5 tons is formed from this type of steel, these nitrides or carbonitrides exist in the form of large pyramidal crystals within the grains and grain boundaries, partly because the alloy elements tend to show segregation and partly because the rate of solidification of the ingot is low. These nitrides and carbonitrides, therefore, do not make any contribution to the increase in the strength of the alloy and, hence, the strength of the steel is not increased substantially by the addition of these carbide formers.
The heat-resisting austenitic steels strengthened by the addition of appreciable amounts of Nb, Ti, Zr and B can form comparatively small ingot having satisfactory strength because such an ingot can easily be treated at a high solid solution temperature. However, it is difficult to form a large ingot from such a steel as will be explained later. The ingot formed from such a steel with the addition of very small amounts of elements such as Nb, Ti, Zr and B exhibits impractically low strength due to the fact that most of these additives is consumed by forming nitrides and carbonitrides. In addition, the creep rupture strength is low particularly in a large-size ingot due to the segregation of the alloy elements.
These nitrides and carbonitrides exist in the grain boundaries near cracks, so that they adversely affect the fatigue life in which the crack propagates from the surface. Thus, the formation of coarse nitrides and carbonitrides is quite inconvenient for the material of steam turbine and valve body which undergoes not only creep but also thermal fatigue due to repeated start and stop of the steam turbine.
High alloys having high Cr and Ni contents, such as Incolloy 800, 15-15N and G18B are known as materials having high strength at high temperature. However, large-size steel products such as steam turbine casing, chemical equipment or the like formed by melting from such a high alloy are unsatisfactory in the strength, toughness, castability, plastic workability and weldability, because of the formation of coarse precipitates as heretofore described.